As portable equipment has spread in recent years, sealed batteries with a compact size, a lightweight, and a high energy density are required for the power supplies of the portable equipment. Among sealed batteries, from the viewpoint of economical efficiency, secondary batteries able to be charged and discharged, such as a nickel-hydrogen storage battery and a lithium ion secondary battery, have been used commonly. Especially, a nonaqueous electrolyte secondary battery represented by the lithium ion secondary battery has been used commonly because of its lighter weight and higher energy density than other secondary batteries.
However, if the secondary battery is overcharged by a longer current supply than normal at the time of charging or is short-circuited by a high current due to improper use, failure of the using equipment or the like, an electrolyte is degraded to generate gas, and an internal pressure of the battery is increased by the gas generation. Furthermore, if the overcharged or the short-circuited state continues, the battery temperature increases rapidly due to the heat generation from the electrolyte and then the sealed secondary battery sometimes explodes suddenly to break the equipment being used. Thus, especially, in the case of the nonaqueous electrolyte secondary batteries, batteries with a safety valve for explosion protection have been used in the related art (see JP-A-6-196138 and JP-A-2003-229177).
The safety valve needs to be activated reliably from the viewpoint of the prevention of equipment breakage, the prevention of a fire accident, and the like. Thus, in the related art, as shown in JP-A-6-196138, a positive electrode plate is placed to oppose a negative electrode plate interposing a separator therebetween, and the whole thereof is wound so as to have a hollow portion at the center to form an electrode body, the electrode body is placed in a battery outer can, and a cylindrical-shaped center pin is placed in the hollow portion of the electrode body, so that the generated gas due to overcharge and the like is led to the safety valve through the center pin placed in the hollow portion of the electrode body. The center pin is arranged in order that a passage of the gas is not blocked by a broken hollow portion caused as the result of a generated gas pressure in the nonaqueous electrolyte secondary battery, since the pressure is exerted on the overlapping direction of the positive electrode plate, the negative electrode plate and the separator.
Furthermore, from the aspects of cost and gas exhaust efficiency, the center pin is generally formed by winding a long thin metal plate in the width direction so as to be tube-shaped. Thus, both edge parts of the thin plate in the circumferential direction of the center pin are not joined and a slit is left between both edge parts. The slit formed on the center pin has roles that not only the passage of the gas generated in the nonaqueous electrolyte secondary battery is secured reliably to activate the safety valve normally, but also a flow of a nonaqueous electrolyte is improved. That is, in the nonaqueous electrolyte secondary battery, since the nonaqueous electrolyte can flow in and out of the center pin through the slit, a flow of the electrolyte in the nonaqueous electrolyte secondary battery is good and ions are exchanged smoothly, so that the battery performance can be enhanced.
However, when the nonaqueous electrolyte secondary battery is dropped down from a high location or an object is dropped on the nonaqueous electrolyte secondary battery, the battery outer can is deformed. In this manner, if the battery outer can is deformed, the center pin placed in the battery also may be deformed. If the center pin is deformed, an edge part facing the slit may stuck to the electrode body to break the separator, and short circuit occurs locally in the electrode body leading to an excessive short circuit current flow. This may cause the nonaqueous electrolyte secondary battery to generate an abnormal heat.
Therefore, in the invention disclosed in JP-A-2003-229177, the edge part facing the slit of the center pin is placed inside of a virtual outer periphery of the center pin, and even if the center pin is deformed by external forces, the slit edge part placed in the inside is difficult to damage the electrode body, so that the inner short circuit does not occur. Furthermore, JP-A-2003-229177 also shows an example that a plurality of window parts is formed on a side face of the center pin, so that the electrolyte flow in the battery is more improved.
However, in accordance with a demand for the improvement of the battery capacity, miniaturization of a space for placing the center pin has been advanced. Thus, in the related-art sealed batteries, when a vicinity of a bottom side face of the battery outer can is heated locally and rapidly, the battery may rarely burst. When meltage of a resin member with a low melting point such as a separator and of aluminum as a substrate of a positive electrode and the like moves by combustion gas, above described phenomena do not cause the problem if the meltage goes through from an opening of the center pin to outside. However, while moving, if the meltage contacts a member with a low temperature, the meltage is rapidly cooled to fix. If the fixed substance exists in the center pin, the center pin is lifted up by a pressure of the combustion gas and the leading end bumps against a sealing member. In such a state, the meltage passed in the center pin has no way out and is fixed at the bottom to clog the center pin, and the gas exhaust to the safety valve side is interfered with, thereby leading to a burst.